Forgot to paste in RST220 description

Author: peterjc

README.rst

@@ -109,7 +109,7 @@ RST216 Multiple roles in interpreted text (both prefix and suffix present; only
 RST217 Mismatch: both interpreted text role suffix and reference suffix.
 RST218 Literal block expected; none found.
 RST219 Inline substitution_reference start-string without end-string.
-RST220 
+RST220 Duplicate explicit target name: "XXX".
 RST299 Previously unseen warning, not yet assigned a unique code.
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